We established criteria for appropriate use of the prostate-specific antigen (PSA) assay and used them to evaluate PSA test utilization at 1 tertiary care institution. During a 6-month period, 2,330 PSA results were reported for outpatients and 95 for inpatients. We reviewed medical records for a random sample of 338 outpatient results (14.51%) and all 95 inpatient results, of which 21% (71/338) of outpatient and 17% (16/95) of inpatient results were inappropriate according to our test utilization criteria. Among outpatients, 52% of tests were done for screening and 19% for monitoring for cancer recurrence. For inpatients, workup for cancer (53/95 [56%]) was the most frequent indication for testing and screening the second (24/95 [25%]). Of tests failing the criteria, 66 (76%) of 87 resulted from excessively frequent and age-inappropriate screening. We assessed the potential effect on clinical outcome if these tests were not performed. Of the 87 tests considered inappropriate, only 1 test result influenced clinical management for patients younger than 75 years. By instituting simple limits on age and frequency, we estimate that 74% (64/87) of the inappropriate tests could have been eliminated.